remove period_helper from non-period reqs

[jbrockmendel]

There are a bunch of other reqs that could be removed (e.g. every direct reference to src/util, all of lib_depends), but at least period_helper it's obvious why it belongs where it does.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @jbrockmendel! Thanks for submitting the PR.

• In the file setup.py, following are the PEP8 issues :

Line 480:80: E501 line too long (84 > 79 characters)

[jreback]

looks fine. ping on green.

[codecov]

Codecov Report

Merging #17531 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17531      +/-   ##
==========================================
- Coverage   91.24%    91.2%   -0.05%     
==========================================
  Files         163      163              
  Lines       49582    49586       +4     
==========================================
- Hits        45242    45225      -17     
- Misses       4340     4361      +21

Flag	Coverage Δ
#multiple	88.99% <ø> (-0.03%)	⬇️
#single	40.2% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/plotting/_converter.py	63.23% <0%> (-1.82%)	⬇️
pandas/core/frame.py	97.77% <0%> (-0.1%)	⬇️
pandas/core/dtypes/concat.py	98.26% <0%> (-0.03%)	⬇️
pandas/core/categorical.py	95.57% <0%> (+0.04%)	⬆️

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[jbrockmendel]

ping.

[jreback]

thanks!